Using 'system*' instead of 'system' in 'guix environment'

[David Thompson]

Hello Guix hackers,

In an effort to finish up a patch to add a --container flag to 'guix
environment', I've encountered a serious problem.  The --exec flag
allows the user to pass an arbitrary command to be run using 'system'.
Unlike 'system*', 'system' spawns a command interpreter first and passes
the command string in.  This is very problematic when using a container,
because there's a very good chance that the command interpreter of the
running Guile process is not mounted inside the container.

So, I think we should switch to using 'system*' instead which will avoid
this hairy issue.  However, it's unclear to me how to make this happen.
I wanted to use 'system*' since I first wrote 'guix environment', but I
couldn't figure out how to make the command line syntax work since each
argument needs to be processed separately instead of being bunched up
into a string.

If the above explanation is confusing, the 'sudo' program provides a
good example of the UI I'm after:

    sudo guile -c '(do-root-things)'

But for now we're stuck with this:

    guix environment --ad-hoc guile -E "guile -c '(do-root-things)'"

Now, we can't actually do exactly what 'sudo' does because 'guix
environment' already recognizes operands as package names, not program
arguments.  Perhaps we can use '--' to separate the package list from
the command to run:

    guix environment --ad-hoc guile -- guile -c '(do-root-things)'

Does that look okay?  Any other ideas?

Thanks,

- Dave